The purpose of this project is to study physiological mediators that influence in vivo and in vitro B and T cell functions. Present studies include: 1) the isolation and characterization of inhibitor(s) of lymphocyte blastogenesis in vitro found in placenta and gravid uteri of mice; (2) the relationship of the inhibitor(s) to observed virus expression in these tissues and their possible participation in allograft (fetal) acceptance; 3) the investigation of events that lead to interactions of virus or viral components with cells, and the consequences of such interactions.